Superconducting thin material and a method for preparing the same

ABSTRACT

A now superconducting material comprising a compound oxide represented by the general formula: 
     
         (Sr,γ).sub.x (La,δ).sub.1-x ε.sub.y Cu.sub.1-y 
    
      O 3-z   
     in which 
     &#34;γ&#34; represents an element of IIa group of the periodic table except Sr, an atomic ratio of γ to Sr being selected in a range between 1% and 90%, 
     &#34;δ&#34; represents an element of IIIa group of the periodic except La, an atomic ratio of δ to La is selected in a range between 1% and 90%, 
     &#34;ε&#34; represents a metal element of Vb group of the periodic table, x, y and z are numbers each satisfies ranges of 0≦x≦1, 0≦y≦1, and 0≦z&lt;1 respectively, and 
     the expression of (Sr,γ) and (La,δ) mean that the respective elements position predetermined sites in a crystal in a predetermined proportion.

This is a divisional of application Ser. No. 224,373, filed July 26,1988, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to a superconducting material and a methodfor preparing the same. More particularly, it relates to a novelsuperconducting material composed of compound oxide having a highercritical temperature and a method for preparing the same.

2. Description of the related art

Under the phenomenon of superconductivity, the perfect diamagnetism isobserved and no difference in potential is observed for all that anelectric current of a constant finite value is observed internally.

The superconductivity can be utilized in the field of power electricapplications such as MHD power generation, fusion power generation,power transmission, electric power reservation or the like; in the fieldof transportation for example magnetic levitation trains, magneticallypropelling ships or the like; in the medical field such as high-energybeam radiation unit; in the field of science such as NMR or high-energyphysics; a high sensitive sensors or detectors for sensing very weakmagnetic field, microwave, radiant ray or the like, or in the field offusion power generation. In addition to the abovementioned powerelectric applications, the superconducting materials can be used in thefield of electronics, for example, as a device using the Josephsondevice which is expected to be a high-speed and low-power consumingswitching device.

The phenomenon of superconductivity, however, is observed only at verylow cryogenic temperatures. In fact, a relatively lower temperature of23.2K which was the critical temperature (Tc) of a superconductorcomposed of Nb₃ Ge have been the top record of the critical temperatureamong known superconducting materials.

This means that liquidized helium (boiling point of 4.2K) is only onecryogen which can realize such very low temperature of Tc. However,helium is not only a limited costly resource but also require alarge-scaled system for liquefaction. Therefore, there had been a strongdemand for another superconducting materials having higher Tc. But nomaterial which exceeded the abovementioned Tc had been found for allstudies for the past ten years.

Possibility of existence of a new type of superconducting materialshaving much higher Tc was revealed by Bednorz and Muller who discovereda new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189]. Itwas also reported in the news paper that C. W. Chu et al. discovered inthe United States of America another superconducting material so calledYBCO type represented by YBa₂ Cu₃ O_(7-x) having the criticaltemperature of about 90K in February 1987. And hence, possibility ofexistence of high-temperature superconductors have burst on the scene.

It had been known that certain ceramics material of compound oxidesexhibit the property of superconductivity. For example, U.S. Pat. No.3,932,315 discloses Ba-Pb-Bi type compound oxide which showssuperconductivity and Japanese patent laid-open No. 60-173,885 disclosesthat Ba-Bi type compound oxides also show superconductivity. Thesesuperconductors, however, possess rather lower transition temperaturesof about 10K and hence usage of liquidized helium (boiling point of4.2K) as cryogen had been indispensable to realize superconductivity.Therefore, the abovementioned new type compound oxides in whichsuperconductivity is realized in liquid nitrogen which is a relativelycheap cryogen will accelerate actual usage of superconductors.

An object of the present invention is to provide a new system ofcompound oxide which possess a higher critical temperature and a methodfor preparing the same.

SUMMARY OF THE INVENTION

A superconducting material according to the present invention iscomposed of a compound oxide represented by the general formula:

    (Sr,γ).sub.x (La,δ).sub.1-x ε.sub.y Cu.sub.1-y O.sub.3-z

in which

"γ" represents an element of IIa group of the periodic table except Sr,an atomic ratio of γ to Sr being selected in a range between 1% and 90%,

"δ" represents an element of IIIa group of the periodic except La, anatomic ratio of δ to La is selected in a range between 1% and 90%,

"ε" represents a metal element of Vb group of the periodic table, x, yand z are numbers each satisfies ranges of 0≦x≦1, 0≦y≦1, and 0≦z<1respectively, and

the expression of (Sr,γ) and (La,δ) mean that the respective elementsposition predetermined sites in a crystal in a predetermined proportion.

A process for producing a superconducting material according to thepresent invention comprise preparing a material powder, compacting thematerial powder and then subjecting the resulting compact to a finalsintering operation, the process is characterized in that said materialpowder is selected from a group comprising following (A), (B) and (C):

(A) a powder mixture composed of powders selected from a groupcomprising

(i) powders of elements Sr, La, Cu, γ, δ and ε and

(ii) powders of compounds each containing at least one of said elementsSr, La, Cu, γ, δ and ε,

(B) a sintered powder obtained by sintering preliminarily the powdermixture (A) and then pulverizing a sintered mass, or

(C) a powder mixture of said powder mixture (A) and said sintered powder(B),

in which, "γ" represents an element of IIa group of the periodic tableexcept Sr, "δ" represents an element of IIIa group of the periodicexcept La, and "ε" represents a metal element of Vb group of theperiodic table, wherein, an atomic ratio of (Sr,γ):(La,δ):ε:Cu in saidmaterial powder satisfies a ratio of x:(1-x):y:(1-y), in which an atomicratio of γ to Sr is selected in a range between 1% and 90%, an atomicratio of δ to La is selected in a range between 1% and 90%, x and y arenumbers each satisfies ranges of 0≦x≦1 and 0≦y≦1 respectively.

The sintered compact obtained by the abovementioned process is composedof so-called quasi-perovskite type compound oxide which is representedby the general formula:

    (Sr,γ).sub.x (La,δ).sub.1-x ε.sub.y CU.sub.1-y O.sub.3-z

in which

"γ" represents an element of IIa group of the periodic table except Sr,an atomic ratio of γ to Sr being selected in a range between 1% and 90%,

"δ" represents an element of IIIa group of the periodic except La, anatomic ratio of δ to La is selected in a range between 1% and 90%,

"ε" represents a metal element of Vb group of the periodic table,

x,y and z are numbers each satisfies ranges of 0≦x≦1,0≦y≦1, and 0≦z≦1respectively, and

the expression of (Sr,γ) and (La,δ) mean that the respective elementsposition predetermined sites in a crystal in a predetermined proportion.

The atomic ratio of γ with respect to Sr can be selected in a rangebetween 1% and 90% but a preferred result is obtained in a range between1% and 50%. Still more, there is such tendency that preferableproperties as superconductors are obtained when the value of x isselected in a range of 0.3≦x≦0.4.

The essence of the present invention resides in that the compound oxideis composed of specified elements and oxygen. This is a unique point ofthe present invention, comparing to the abovementioned known new typesuperconductors composed of four elements. Namely, the compound oxideaccording to the present invention, two pairs of elements each pair ofwhich belongs to the same group of the periodic table. It is though thatan advantageous result of the present invention is obtained when each ofthese elements occupies a predetermined site in the crystal in apredetermined proportion.

We found that precise control of the following factors is indispensablein order to obtain a sintered compound oxide possessing superiorsuperconducting properties:

(i) particle size of the material powder,

(ii) temperature during the preliminary sintering operation,

(iii) particle size of the preliminary sintered and pulverized powder,and

(iv) temperature during the final sintering operation.

In fact, if an average particle size of the material powder exceed 10μm, a fine particle which is satisfactory in uniformity can not beobtained easily even after such material powder is subjected to thepreliminary sintering operation. Therefore, the average particle size ofthe material powder is preferably less than 10 μm.

In case that a sintered powder (B) which is prepared by preliminarysintering of the powder mixture (A) is used as the material powder, thepreliminary sintering is preferably carried out at a temperature whichis higher than 700° C. but is not higher than the lowest melting pointof any compound which is contained in the powder mixture (A) for aduration of from 1 to 50 hours and preferably in an atmospherecontaining oxygen gas of 10⁻³ to 10² Torr. It is also preferable that,after the preliminary sintering complete, the resulting sintered mass iscooled at a cooling rate which is not slower than 1° C./min but nothigher than 1,000° C./min. A particle size of the pulverized powderobtained from the preliminary sintered mass influence directly upon aparticle size of a crystal which is obtained after the final sinteringoperation, so that the preliminary sintered mass is preferablypulverized to powder having an average particle size of less than 5 μm.Such fine particles increase the area of grain boundaries which is oneof critical factors for realizing the superior superconductor which isparticularly improved in the critical temperature of superconductivity.Pulverization or reduction of the preliminary sintered mass to less than1 μm is not only economical or practicable but also increase possibilityof contamination of the material powder. Therefore, the average particlesize of the preliminary sintered powder is preferably adjusted to arange between 1 μm and 5 μm.

A series of operations of preliminary sintering, pulverization andcompacting is preferably repeated for several times to homogenize thematerial powder.

The sintering temperature is one of the critical factors in the processfor producing the superconduting compound oxide, so that the sinteringtemperature is controlled and adjusted to a temperature at which thesintering proceed in a solid reaction without substantial fusing of thematerial powder and excessive crystal growth is not occur in theresulting sintered compound oxide. In fact, the sintering temperatureshould not exceed the lowest melting point of any component in thematerial powder such as the powder mixture, the compound powder or thepreliminary sintered powder. To the contrary, satisfactory sintering cannot be effected if the sintering temperature is too low. Therefore thesintering must be carried out a temperature which is higher than 700° C.Duration of sintering operation is generally for 1 hour to 50 hours inactual practice, although longer sintering time is preferable.

The abovementioned preliminary sintering operation should be alsocontrolled precisely in the same manner as above because of the samereason.

According to a preferred embodiment, the sintered compound oxide isfurther heat-treated in order to homogenize the crystal structure. Thisheat-treatment improve the critical temperature and reduce remarkablythe discrepancy between the terminal temperature of phase change whereperfect zero resistance is observed and the critical temperature.

This heat-treatment is preferably carried out in an oxygen containingatmosphere at a temperature of 500° to 900° C. Under this condition ofheat-treatment, the crystal structure of sintered compound oxide isstabilized and the oxygen deficient perovskite structure which isdesired for realizing the superconductivity is obtained, resulting inthat the lower critical temperature where perfect zero resistance isobserved become much higher and a lasting and stable superconductor isassured. If the temperature of heat-treatment is not higher than 500° C.the abovementioned effect is not obtained or it takes longer time beforerealization of the objective crystal structure. To the contrary, if thetemperature of heat-treatment exceed 900° C., the abovementionedperovskite type crystal structure is lost.

The superconducting material according to the present invention can beused in a form of a sintered mass or article as it is and is also usedin a form of a powder which is prepared by pulverizing the sinteredmass. This powder-formed superconducting compound oxide can be used forproducing a superconducting wire or the like. For example, thesuperconducting compound oxide powder according to the present inventionis compacted in a metallic pipe which is then drawn into a fine wire oris mixed with suitable binder such as polybutylbutylal (PVB) to preparea paste which can be molded into a desired configuration or which iscoated or applied in a desired pattern. The resulting wire and the pastemolded or coated are then sintered finally.

When the superconducting compound oxide according to the presentinvention is used in the form of a paste, it is preferable to heat thecoated or molded paste at 400° to 700° C. in air before the finalsintering operation in order to remove the binder previously.

When a self-supporting article is molded with the paste, it ispreferable to select a thickness of the paste to be molded to less than1.2 mm and when a thick film is coated on a support, the thickness of alayer of paste to be coated on a support is adjusted to less than 0.6mm. In practice, since a pre-form of paste molded or coated by a doctorblade coating technique or extrusion technique shrink during the finalsintering stage, the dimension of the final product becomes smaller.Therefore, when the paste is shaped into a form of a tape or wire or iscoated into a thick film, their thickness or diameter of paste molded orcoated is preferably controlled to less than 1.2 mm or 0.6 mmrespectively.

The superconducting material according to the present invention can beformed into a thin film by the conventional physical vapor deposition(PVD) technique such as vacuum deposition, sputtering, ion-plating ormolecular beam epitaxial growth technique in the presence of oxygen gas.In this case, the abovementioned sintered mass or block is used as avapor source or target which is evaporated in a vacuum chamber toproduce a thin film deposited on a substrate. Namely, the vapor sourcemay be a block prepared by sintering the material powder such as (i)powders of metal elements of Sr, La, Cu, γ, δ and ε (elements γ, δ and εhave the same definition as above) as they are, (ii) a powder mixture ofcompounds of these elements or (iii) their combination. The vapor sourcemay be a powder which is obtained by pulverizing the sintered block. Theproportion or atomic ratio of the elements in the vapor source isselected in such manner that the desired atomic ratio of the elementsare realized in the deposited thin film in consideration of vaporizationrate or sputtering rate or the like. The oxygen pressure in the vacuumchamber should be controlled so that the partial pressure of oxygen gasis adjusted to a range between 10⁻⁶ and 10² Torr. The substrates onwhich the thin film are deposited are prefarably those that have similarcrystal structure or lattice constant and may be a single crystal ofMgO, sapphire or SrTiO₃. Desirably, the superconducting thin film isdeposited on a {001} plane or {110} plane of a single crystal of MgO orSrTiO₃ to improve the critical current density (Jc) owing to ordering ofcrystal to c-axis.

In this case of thin film also, the abovementioned heat-treatment of thedeposited thin film is very effective. The heat-treatment is carried outat a temperature of 500° to 900° C. for more than 1 hour in oxygencontaining atmosphere in which the partial pressure of oxygen isadjusted to 10⁻³ to 10² Torr. After the heat-treatment, the resultingthin film is cooled slowly at a cooling rate of less than 10° C./min.

The superconducting material according to the present invention show avery high critical temperature comparing to the conventional materials.This might come from their characteristic feature of the composition andtheir uniform and fine crystal structure which is assured by the presentprocess.

The novel superconducting materials according to the present inventionhave improved stability and a high critical temperature, so that theycan realized superconductivity by means of relatively cheaper cryogen ina small liquefication system and hence they can be utilizedadvantageously in applications of superconducting wire, rod, parts suchas magnets, thick film or thin film devices, such as Matisoo switchingelements or Josephson device, Anacker memory device, a variety ofsensors or Superconducting Quantum Interference Device (SQUID).

Now, embodiments of the process according to the present invention willbe described by Examples, but the scope of the present invention shouldnot be limited thereto.

EXAMPLES

Powders of commercially available CuO and carbonate powders of Sr, Ca,La, Sm, Yb and Bi, namely SrCO₃, CaCO₃, La₂ (CO₃)₃, Sm₂ (CO₃)₃, Yb₂(CO₃)₃, (BiO)₂ CO₃ are mixed in such an atomic ratio that satisfies thefollowing general formula:

    (Sr,Ca).sub.x (La,δ).sub.1-x Bi.sub.y Cu.sub.1-y

in which, δ represents Sm or Yb, the value of x and y are shown in Table1, and the proportion (%) of Ca with respect to Sr and the proportion(%) of δ with respect to La are also shown in the Table 1.

The respective powder mixtures are pulverized in a ball mill to bereduced to an average particle size of 4 μm and then sintered at 750° C.for 15 hours. The resulting sintered mass or cake is pulverized again.The steps of sintering and pulverization are repeated two times underthe same condition as above. The finally obtained powder of less than 4μm is charged in a rubber mold and compressed hydrostatically under apressure of 1.5 ton/cm² to obtain tablets of 4×10 ×30 mm. The tabletsare sintered at 950° C. for 8 hours in air.

Resistance is measured on the tablets on which electrodes are securedwith silver electroconductive paste to determine the criticaltemperature.

Measurement of an upper critical temperature Tc and a lower criticaltemperature Tcf is effected by a conventional four probe method.Temperature is measured by a calibrated Au(Fe)-Ag thermocouple.

The result are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Sample (Sr,Ca)x Bi      Ca         δ                                    No     (x)      (y)     (%)  δ                                                                             (%)  Tc   Tcf                              ______________________________________                                        1      0.66     0.11    10   Sm    30   148  129                              2      0.66     0.11    10   Yb    30   130  116                              ______________________________________                                    

What is claimed is:
 1. A superconducting thin film disposed or asubstrate comprising a compound oxide represented by the generalformula:

    (Sr,γ).sub.x (La,δ).sub.1-x ε.sub.y Cu.sub.1-y O.sub.3-z

in which γ represents an element of IIa group of the periodic tableexcept Sr, an atomic ratio of γ to Sr being selected to be in a rangebetween 1% and 90%, δ represents an element of IIIa group of theperiodic table except La, an atomic ratio of δ to La being selected tobe in a range between 1% and 90%, ε represents a metal element of Vbgroup of the periodic table, x, y and z are numbers in the ranges of0≦x≦1, 0≦y≦1, and 0≦z≦1 respectively, and (Sr, γ) and (La, δ) mean thatthe respective elements position predetermined sites in a crystal in apredetermined proportion.
 2. A superconducting thin film as set forth inclaim 1, wherein the atomic ratio of γ to Sr is selected to be in arange between 1% and 50%.
 3. A superconducting thin film as set forth inclaim 1, wherein said superconducting thin film has perovskite type orquasi-perovskite type crystal structure.